When electronic devices are separated by a physical distance that may be greater than a few percent of an electrical wavelength at frequencies of interest, a transmission line is generally used to exchange signals. A transmission line with a characteristic impedance matched to an output impedance of a line driver at its transmitting end and to the impedance of a load at its receiving end is generally employed to avoid signal reflections that can be produced at either end of the line. Signal reflections generate “standing waves” on a transmission line, which can compromise the error rate of signal reception. Although transmission lines are typically viewed as coaxial cables and other similar structures, a patterned path on a printed wiring board or in an integrated circuit can also operate as a transmission line structure.
A signal such as a digital signal is generally transmitted across a transmission line “differentially.” In differential signal transmission, the voltage of one conductor of the transmission line is increased, and the voltage of the other conductor is reduced by a corresponding amount. For example, to transmit a signal with a first signal sense, e.g., a binary “1,” one conductor of a transmission line might be raised from 1.0 volt to 1.5 volt, and the other conductor reduced from 1.0 volt to 0.5 volt. The opposite voltage changes would be applied to the transmission line conductors to transmit a signal with a second signal sense, e.g., a binary “0.” By transmitting a signal with symmetrical voltage changes, the transmission line does not transmit a common-mode voltage component to its receiving end. Accordingly, a virtual ac ground can be created at the receiving end of the transmission line, which, among other issues, avoids the need to match a common-mode impedance at either end of the transmission line. The technique of transmitting signals with symmetric voltage changes allows data to be transmitted at a high rate with minimal interference from noise that may be induced onto a wire pair by external electromagnetic effects. In addition, reference potentials such as ground references at the transmitting and receiving ends of the transmission line can be at different potential levels using such a differential signaling approach. Closely located transmitters and receivers sharing a common ground reference, however, can avoid the need for such differential signal transmission.
To avoid reflections for differential mode signals at either end of a transmission line requires that both a transmitter and receiver terminate the transmission line with an impedance that matches the transmission line (differential-mode) characteristic impedance. The characteristic impedance of a transmission line is related to electromagnetic energy stored per unit length in the line's inductance and capacitance, and is an inherent property of the physical dimensions and materials forming the line. The need to match transmission line impedances generally introduces a power-dissipating element in a line driver at the transmitting end of the line. Such a power-dissipating circuit element detracts from battery life in portable equipment, and contributes to size, cost, and reliability issues in equipment that may be powered from local ac mains.
Thus, there is a need for transmission line driving arrangements and products using arrangements capable of coupling a differential signal to a transmission line with an impedance matched to the characteristic impedance of the transmission line with minimal power dissipation.